Lecture 4 Notes  October 6-7, 2004

The climatic influence of clouds is a combination of their influence on the planetary albedo and the IR budget. These two influences depend a great on the cloud properties. In class I described two extreme clases of clouds - high thin and low thick.

cirrus clouds                                       stratus cloud

The schematics show cloud influences on the solar and IR fluxes for thin high clouds (left) and low thick clouds (right). On the left we see much of the solar flux passes through the thin high cloud and the IR emitted from this cloud is relatively small compared to Earth's surface. Thus the outgoing IR flux escaping from the planet is relatively low (and made so by the thin high cloud). In contrast the thick low cloud blocks a lot of solar radiation and emits an IR flux similar to that of the surface. Consequently the thick cloud mostly reduces absorbed solar flux but has little influence on the outgoing longwave radiation. Overall the thin high cloud has a warming influence on the surface and the thick low cloud has a cooling influence compared to cloud-free conditions. 

Remember clouds are fairly good blackbody emitter/absorbers but their height in Earth's atmosphere has a strong influence on the strength of their greenhouse effect. High clouds tend to have a stronger greenhouse effect. This is analogous in some ways to a thick versus thin atmosphere (think greenhouse effect of Venus compared to Mars ).

We can show the greenouse effects of clouds from simple energy balance where we assume that the atmosphere has no gases and just one cloud:
Low Cloud GHE

We must consider clouds influence on the planetary albedo to determine whether a cloud has an overal cooling or warming influence on the climate. Again we ignore all other gases and clouds and just consider this one cloud alone:
High Cloud Energy Balance
low cloud energy balance
You can see for the characteristics I chose, the low cloud would cause a net loss of IR radiation to space that is greater than the net incoming solar (i.e., absorbed solar radiation). Thus the low cloud would have a cooling effect, and eventually the air and surface temperature would decrease. In contrast the high cloud would have a warming effect for the parameters I chose and the air and surface would have to warm. The temperature will adjust until the planet reaches a new equilibrium.

A change in clouds can either cool or warm the planet depending on the type of clouds. This is one reason why the response to increasing levels of greenhouse gases is hard to predict. We must know how the clouds will change because they have a strong influence on climate change.

The average effect of clouds (not the change in clouds in the future) is easier to understand. We think, based on the average characteristic of clouds, that clouds have an overal cooling effect on the average climate today. Hence if Earth lost all its clouds, the planet would warm.

Please read the notes on convection. I'll include a few a few illustrations here.

Adiabatic Expansion
The lapse rate is defined to be the temperature decrease with height. It is defined to be positive for the typical atmospheric condition of a temperature DECREASE with height. When parcels of air move up or down they must follow the dry adiabatic lapse rate, provided they exchange no heat with their surroundings, which is normally a good approximation, and there temperature has not reached the saturation point for the concentration of vapor contained in the parce. The real atmosphere does not necessarily follow the dry adiabatic lapser rate because the real atmosphere is also influenced by IR absorption and emission and condensation.

If the real atmosphere should somehow develop a lapse rate that is larger than the dry adiabatic lapse rate, it is called unstable and it will adjust very quickly and sometimes violently. These are conditions that may lead to thunderstorms and tornadoes.
unstable atmosphere
If the real atmosphere instead has a lapse rate that is smaller than the dry adiabatic lapse rate, it is stable.

So far I have only talked about the stability of a dry atmosphere. Adding condensation makes it a little more complicated but the concepts are mostly the same.

This brings us to the end of Chapter 3 and we return to the feedback loops we brought up in chapter 2 again. I talked about a water vapor feedback loop that is positive:
water vapor fb
Remember I said that something else prevented a runaway greenhouse effect on Earth.
This something else is the strong negative feedback from IR radiation emitted by Earth's surface.
It is sometimes called the most fundamental feedback on Earth. It is so fundamental that sometimes we forget that it is a feedback at all: Recall that it defined the effective radiating temperature of the planet.
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Contact the instructor at: atms211@atmos.washington.edu

Last Updated: 9/29/2004